The evolution of wireless devices has dramatically increased the availability and adoption of voice and data services. This has created a significant demand for improved wireless devices and in turn has complicated the design considerations for transmitters/receivers and antennas that can be used in wireless devices.
Until the advent of mobile handsets, portable wireless terminals and other wireless devices, antenna design was based often purely on technical radio specifications such as gain, bandwidth, and polarization, for example. Demand for reduced size, increased power efficiency and the requirement to meet ever more stringent regulations that limit emission of and/or exposure to radiation have emphasized a number of additional aspects in the design of antennas for wireless devices. Specifically antenna efficiency and a reduction of user irradiation by a wireless device have become important design considerations for transmitter stages for use in wireless devices.
When used for transmission purposes, an antenna may emit substantial amounts of electromagnetic radiation. The electromagnetic radiation emitted by the antenna is characterized by electric and magnetic field components as well as polarizations of the electromagnetic field components. The electromagnetic field forms a radiation pattern depending on distance and/or orientation relative to the antenna. The electromagnetic radiation that is emitted into the environment may interact with other elements including plants, animals, persons and, for example, users of a wireless device and may be partially or substantially absorbed by one or more elements. Other elements, furthermore, may have an effect on the radiation pattern. Absorption of the radiation by the elements may depend on the composition and shape of each element.
Exposure to electromagnetic radiation may depend on distance and orientation of an element relative to the antenna. For example, for a user of a wireless device, exposure to electromagnetic radiation from a transmitting wireless device may be determined by proximity of the user to the wireless device and the solid angle that a user occupies relative to the wireless device and/or the antenna. Specific absorption of radiation (SAR) is the measure of power absorbed by a volume (typically 1 gm equivalent to 1 cc) of tissue comprising the body of interest due to incident RF radiation upon that volume within the tissue. Specific use scenarios of a wireless device by a user may result in different SAR levels in different regions of the user's body. For safety reasons, SAR limits for different regions of the body are prescribed in many jurisdictions.
Wireless devices are subject to SAR limits in many countries to ensure that device users are not exposed to unacceptable irradiation levels. Regulations may prescribe different SAR exposure limits for different regions of the body. SAR is determined by the radiation absorbed by a person and is typically defined in terms of power absorbed per mass of tissue. SAR can depend on a number of aspects including, for example, the position and orientation of the wireless device relative to the person, the field emission characteristics of the wireless device, the transmitting power and frequency, the polarization of the radiation, as well as ambient elements which may interact with the radiation emitted by the wireless device.
As the operating frequencies for wireless devices are usually fixed and cannot be independently controlled, known solutions to limit SAR build upon control of one or more of the other aspects. Although, depending on the wireless communication protocol, transmission power and consequently user irradiation may be reduced in close proximity to wireless base stations, means are required to limit SAR even at full transmission power of a transmitter. Designing wireless devices with good transmitter performance and sufficiently low SAR has become increasingly challenging.
Known solutions for limiting SAR include intermittent transmission thereby reducing time averaged data transmission rate, removing the device or at least the transmitter from the user, reducing transmission power with decreasing distance from a base station, controlling the near-field radiation emission characteristics, as well as configuring an antenna system so that it emits radiation primarily in predetermined directions away from a user. Known solutions, however, require sophisticated antenna designs and/or mechanisms to control the operation of an antenna in a wireless device, which in turn may require wireless devices with complex hardware and/or software.
A solution that seeks to control radiation emission characteristics includes United States Patent Application Publication No. 2007/0210970, which relates to a system for reducing local SAR by suppressing concentration of the current flowing through an antenna element provided through two or more feed points. Furthermore, United States Patent Application Publication No. 2009/0009407 discloses generating resonance in a multi-band antenna having at least two feeding ports, to improve mute performance, reduce SAR, and prevent reduction in call performance caused by a user's body and hand. In addition, U.S. Pat. Nos. 6,762,724, 6,552,689, 6,509,882 and 6,031,495 relate in general to wireless communication devices with reduced SAR. However, current solutions for controlling SAR can be ineffective in meeting regulatory requirements. Therefore there is a need for a new method and apparatus for controlling radiation characteristics of an antenna that overcomes one or more of the problems in the art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.